1. Field of the Invention
The present invention relates to video recording and playback, and more particularly relates to video recording discs and their playback.
2. Background Art
The videodisc is a storage medium for video information, on which video signals are recorded as spiral or circular tracks of indicia which are readable by way of electrical, optical, or other means. Videodiscs, in particular optical videodiscs which are readable optically by way of an imaged beam of laser light, offer enhanced playback flexibility as compared with video tape. To cite just one example, a videodisc player can be programmed to "search" for a particular frame or field of video, and that search can be effected quite rapidly as the imaged beam of laser light is easily moved in a radial direction across the full span of a disc much more rapidly than a video tape can be cycled from beginning to end even in a fast forward or fast reverse mode.
One limitation imposed by the videodisc, however, at the present level of technology, is in the amount of video information which can be stored on one "side" of a disc. Current optical videodisc standards provide for 54,000 circular or spiral tracks on a disc, with the innermost, limiting track carrying no more than two fields of video. Constant angular velocity ("CAV") discs, which are rotated at a constant angular velocity as they are played, carry two fields of video in each of the 54,000 tracks. Constant linear velocity ("CLV") discs, which are played at a varying angular velocity such that the linear velocity of the reading spot of light along any track on the disc is constant, carry a varying number of fields of video on each track, varying from two on the innermost track to six on the outermost track. The maximum permissable play time for a CLV disc is approximately one hour per side, while for a CAV disc the play time is limited to approximately one half hour per side.
Arrangements have been proposed to extend the play time of videodiscs. For example, U.S. Pat. No. 3,908,080, entitled "Method of Making an Extended Play Videodisc Record", which issued on Sept. 23, 1975 to Kent D. Broadbent, describes a system wherein only one of the video frames of a consecutive plurality of frames is recorded on a videodisc. The sound associated with the recorded video frame and with the omitted video frames is multiplexed and written on the disc track along with the recorded frame. On playback, the recorded frame is read repeatedly by the player a sufficient number of times to substitute for the omitted frames and each repeat is accompanied by the sound corresponding to the omitted frames, in proper sequential order. The system disclosed in the '080 patent provides a system by which the play time of a videodisc may be extended by a factor of two, three or more times. The system lends itself particularly well to applications wherein one frame is recorded on each spiral track revolution of the videodisc. In this way, the mechanics of repeating a single frame is rendered relatively simple, utilizing the standard technique of "freeze frame" videodisc playback.
However, the '080 patent system has limitations. For example, since whole frames are dropped, the maximum motion update rate is 15 per second. This gives marginal motion fidelity. In addition to this, whenever Field 1 and Field 2 of the frame being repeated are different, the transition between the fields is seen the same number of times the frame is repeated, exaggerating the jerking motion.
The present invention solves these problems, while permitting up to 2/3 of the video signals to be omitted, thus giving three times the normal play time for a videodisc.